Sea levels have fluctuated dramatically in geologic times. It was 2-6 m above the present level during the last interglacial period, 125,000 years ago, but 120 m below present levels during the last Ice Age, 20,000 years ago. In the last 100 years it has increased by 10-25 cm. Sea level could rise 40 to 65 cm by the year 2100, due to predicted greenhouse-gas-induced climate warning. Such a sea level rise would threaten coastal cities, ports, and wetlands with more frequent flooding, enhanced beach erosion, saltwater encroachment into coastal streams and aquifers.
However, future sea level is very difficult to predict, because not enough is known about how the ice sheets in Greenland and Antarctica will react to global warming. Furthermore, local sea level is affected by many regional processes, including tides, ocean currents and geographically varying land movements. These Earth motions are caused by ongoing adjustments of Earth’s crust to the removal of the former ice sheets, tectonic deformation, subsidence of river deltas under sediment loads, and extraction of underground water, oil, or natural gas near the coast.
A comparison of the tide-gauge records and radiocarbon-dated geologic data from four widely separated regions, spanning a broad range of geologic settings, indicate modern sea-level trends to be consistently 1-1.8 mm/year higher than those derived from long-term geologic data. This result implies a recent acceleration of sea-level rise relative to the last few thousand years.
The nationwide impacts of sea level rise are:
Saltwater intrusion resulting in increased salinity
Floods and flood damages
Threats to coastal wetlands
Threats to agriculture
Apart from the above direct effects, there are some indirect effects of sea level rise, which sometimes may be more significant than direct effects in the future.
Though the consequences of sea rise are not alarming presently, the time is ripe for taking adaptive actions as the effects of sea level rise are becoming inevitable because of green house gas emissions, the inertia of the oceans, and the economy’s current dependence on fossil fuels. Any corrective action at this stage would be welcome. This includes decision to rebuild after a coastal disaster. Again, society can save money by preparing for sea level rise, but such preparation is impossible without reasonably reliable projections of how much the sea might rise.